Power semiconductor devices are generally used in applications which require the manipulation of high levels of current. These well known applications include electric motors for industrial use, such as heating, cooling, and ventilation systems, electric vehicles, switch mode power supplies, and power factor correction applications. The current that must be handled by each power module can be as high as 1200 Amps. This necessitates the use of heavy power leads to route current in and out of the power module. The leads are generally attached to a metal circuit pattern with solder paste, which is reflowed in a belt furnace or on a hot plate.
There are several inherent problems with this type of lead design and construction. By design, the solder joint between the heavy copper power lead and the thinner metallized circuit pattern is not a reliable joint. Solder fatigues very easily and any stresses applied to the power lead after it is attached quickly create cracks in the joint and can cause failures. The heavy bus bars to which power modules are mounted can create a large twisting moment on the power leads. If the stresses are not properly relieved, the stresses are transferred to the solder joint and will contribute to crack propagations during thermal cycling. Another problem with this type of power lead is assembly. These leads are generally difficult to manipulate in volume production and often require complicated fixturing to keep them in place while the solder paste reflows. Variations in tolerances in the leads or fixtures complicate the assembly further and often result in a less than ideal joint.